1. Field of the Invention
This invention relates to light beam scanning methods and apparatus, and more specifically, to methods and apparatus for minimizing or eliminating the effects of scan line spacing error.
2. Description of the Prior Art
Many reproduction apparatus, such as electrostatographic printers or copiers, use a beam scanning process for exposure of a photosensitive member. Often called output scanning, the process provides for modulation of a light beam as the beam is moved relative to the surface of the photosensitive member. An image composed of scan line exposures is then provided. The image thus formed is developed and transferred to a receiver.
Image degradation occurs when the spacing of scan line exposures is non-uniform. Scan line spacing error creates a density banding effect, which is a visible and highly-objectionable variation in image density between recorded raster lines in the final print. The density banding may be sufficient to cause image degradation that is immediately visible in a monochromatic print.
If successive images are to be superposed on a single receiver to form a multicolor image, very precise registration of the images governs the quality of the multicolor image. Similarly, successive images may be individually used as color separation masters in a xeroprinting, lithoprinting, or other print-making apparatus, wherein the accuracy of image registration is often critical to final print quality. Scan line spacing errors in the various print layers of a composite multicolor print can produce undesirable color shifts and loss of detail.
Some scan line spacing error is attributable to a variation, or "flutter", in the transport speed of the photosensitive member. This variation is typically due to transport rate errors in the transport system for the photosensitive member. Nearly all types of electrophotographic copiers and printers experience some flutter due to friction or mechanical inaccuracy in the transport system. Another source of flutter is the drag imparted to the photosensitive member as it is acted upon by the toning, transfer, and cleaning stations.
Output scanners have been considered for use in replacing the exposure station of an optical input copier or printer. The conversion is difficult because the amount of tolerable flutter is inherently greater in an optical exposure station than in a scan exposure station. Optical input copiers generally flash expose the entire image frame in roughly one-tenth of a millisecond. A scanning exposure station, however, will write the image one pixel a time in numerous line scans to complete the image in roughly a second. Thus, a given amount of flutter that is acceptable in a flash exposure will cause undesirable image banding artifacts during a one-second scanning exposure.
Scan line spacing error in an output scanner also occurs due to imprecise timing of line exposures relative to the movement of the photosensitive member. Beam scanning must be coordinated with the speed and position of the photosensitive member and with the modulation, or illumination control, of the beam. Successive scans provided by a beam deflector construct an entire image on the web, but the web rotates asynchronously with respect to the location of the light beam on the scan line. Hence, when the web is in a position to receive the line scan exposure, the beam deflector may not be in the proper position for starting a scan line. The exposure must be delayed to allow the beam deflector to move to the requisite position for initiating a scan line. In the worst case, the beam deflector will have just passed the requisite position when the exposure is due. Significant misregistration of the exposure can then occur.
In U.S. Pat. No. 4,779,944, issued in the name of Ritter et al., an acousto-optical diffraction grating modulator is used to compensate for minor errors associated with a photosensitive member transport system. The modulator deflects a laser beam to compensate for positional errors associated with a xerographic drum rotation mechanism.
However, acousto-optical modulators can be relatively complex and expensive. An acousto-optical modulator typically requires a radio frequency generator which applies an amplified, high frequency signal to an acoustic transducer. The transducer then launches acoustic waves in the acousto-optical cell to create a diffraction wave grating. The radio frequency generator is subject to instability and requires frequent realignment of the amplitude and frequency of its output. Moreover, the rise time of an acousto-optical modulator is dependent upon the diameter of the modulated light beam. Hence, to achieve high speed from an acousto-optical modulator, it is necessary to reduce the dimension of the optical beam that is crossed by the acoustic wave. This reduction is undesirable, as additional beam-shaping optical components are then required to restore the beam to a proper condition.